This invention relates to ferromagnetic devices, such as ferroresonant devices, constant voltage transformers, reactors, and the like, having a magnetic core which contains a shunt in which variation in the length of an air gap will change the operating characteristics of the device. The invention relates especially to means of providing different effective air gaps and thereby obtaining different operating characteristics.
Such devices are exemplified by constant voltage transformers of the general type shown in Sola U.S. Pat. No. 2,143,745 of Jan. 10, 1939. In such constant voltage transformers, operation depends on the presence in the magnetic core structure of one or more magnetic shunts which include air gaps, and the particular operating characteristics of the transformer depend on the effective air gap lengths in the shunts.
The magnetic core structures of such transformers are commonly made from stacks of laminations, such as E-I laminations, which form spaced main core legs on opposite sides of a window. The shunt legs of the cores are preferably and commonly made as stacks of separate shunt laminations of a length to extend across the window, but shorter than the width of such window by a predetermined amount so as to define an air gap of predetermined length. Sets of laminations are commonly stamped from sheet stock in a low-scrap E-I pattern in which the I-laminations and the shunt laminations are formed from stock in the windows of the E-laminations.
The sheet stock used is preferably grain-oriented and has a preferred direction of flux flow. The laminations are cut so that the direction of grain orientation and flux flow extends lengthwise of the center and side legs of the E-lamination, and lengthwise of the I-lamination. The shunt laminations are preferably stamped with such preferred direction running lengthwise therein.
It is common practice to punch bolt holes at the ends of the back leg of each E-lamination and at the ends of the I-lamination, and to punch a single rivet hole through the exact center of the shunt laminations, so that the stacks of laminations can be fastened together when assembled in a transformer.
In such prior practice, if it is desired to have a different air gap in the shunt of the laminated core structure, it is necessary to produce shunt laminations of a different length. In practice and for economical production, core laminations are commonly punched in complete sets from strip stock in a progressive die, so that to produce shunt laminations of a different length in such case requires the making of a new progressive die for the entire set of laminations. Alternatively, the different length shunts can be cut to precise length from strip stock slit to the required shunt width. Either method is expensive and requires extra tools and extra inventory.
It is also known, as from Smith U.S. Pat. No. 3,456,223 of July 15, 1969, that the performance of constant voltage transformers under varying loads is improved by stacking the shunt laminations in an offset arrangement, with a center bundle of laminations displaced longitudinally to abut one leg of the magnetic core structure and with side bundles of the laminations displaced in the opposite direction to engage the opposite leg of the magnetic core structure. With such offset arrangement, at low load current levels the juxtaposed bundles of shunt laminations provide an essentially low-reluctance path which extends from the side bundles to the center bundle and which contains substantially no air gap at either end, whereas at higher load current levels the flux path extends directly through the individual bundles and across the air gaps at their ends. In the Smith patent, the staggered relationship of the center and side bundles of laminations is obtained by inserting a gap spacer at the end of at least one bundle of laminations to position the bundles in offset relationship.
The present invention provides a means for providing different effective air gaps and correspondingly different operating characteristics while using identically the same core lamination sets, including the same identical shunt laminations. The lamination sets for devices having different effective air gaps and correspondingly different characteristics may be made in the conventional economical manner on the same progressive dies, and may be in a low scrap pattern. The invention also provides the same benefits as the disclosure of the Smith patent.